There are several types of microphones and related transducers, such as for example, dynamic, crystal, condenser/capacitor (externally biased and electret), etc., which can be designed with various polar response patterns (cardioid, supercardioid, omnidirectional, etc.). Each type of microphone has its advantages and disadvantages depending on the application.
Micro-Electrical-Mechanical-System (“MEMS”) microphones, or microphones that have a MEMS element as the core transducer, have become increasingly popular due to their small package size and high performance characteristics (e.g., high signal-to-noise ratio (“SNR”), low power consumption, good sensitivity, etc.). However, due to the physical constraints of the microphone packaging, the polar pattern of a conventional MEMS microphone is inherently omnidirectional, which can be less than ideal for wideband applications, such as, e.g., recording studios, live performances, etc.
More specifically, MEMS microphones effectively operate as “pressure microphones” by producing an output voltage proportional to the instantaneous air pressure level at the transducer location. For example, MEMS microphone transducers typically include a moving diaphragm positioned between a sound inlet located at a front end of the transducer for receiving incoming sound waves and a rear acoustic chamber that has a fixed volume of air and is formed by a housing covering a back end of the transducer. Changes in air pressure level due to incoming sound waves cause movement of the diaphragm relative to a perforated backplate also included in the transducer. This movement creates a capacitance change between the diaphragm and the backplate, which creates an alternating output voltage which is sensed by an integrated circuit (e.g., Application Specific Integrated Circuit (“ASIC”)) included in the microphone package. As will be appreciated, because the housing (e.g., enclosure can) covers the back end of the MEMS transducer, it blocks rear acoustic access to the moving diaphragm of the MEMS transducer. As a result, the MEMS microphone receives sound only through the sound inlet at the front end of the transducer, thus creating an omnidirectional response.
Accordingly, there is a need for a MEMS microphone with a directional polar pattern that can be isolated from unwanted ambient sounds and is suitable for wideband audio and professional applications.